In general, carbon fibers are produced industrially mainly from rayon, PAN (polyacrylonitrile) or pitch. PAN, however, has disadvantages in that it is expensive and the carbonization yield is low. On the contrary, pitch is inexpensive and thus is attractive from an economic standpoint. Of pitch materials, an isotropic pitch cannot provide high quality carbon fibers because of its poor orientation. On the contrary, carbon fibers produced from an optically anisotropic pitch called a mesophase pitch have a highly oriented structure in which carbon crystallites are preferentially aligned parallel to the fiber axis and thus have excellent mechanical characteristics, that is, high strength and high modulus of elasticity.
Therefore, extensive investigations are being made on the production of a mesophase pitch as a raw material for production of high quality carbon fibers, from a catalytic cracking residue of oil, a naphtha tar pitch or a coal tar pitch. It has been confirmed by many experiments that molecules composed mainly of polycondensed aromatics are orientated in the direction of the fiber axis and thus high quality carbon fibers can be obtained from the mesophase pitch. The mesophase pitch, however, has disadvantages in that the viscosity is high and thus the softening point is high because of the interaction of polycondensed aromatics. For this reason, various investigations have been made to improve the spinning properties of the mesophase pitch by lowering its softening point. When, however, a complicated mixture such as a petroleum pitch or a coal tar pitch is used as a raw material and is subjected to thermal modification for the purpose of developing a polycondensed aromatic structure, the resulting pitch inevitably has a continuous and wide distribution of molecular weight. Therefore, if such a complicated non-homogenous mixture is used as a raw material, it is difficult to control the chemical structure of the product pitch and thus to selectively produce a pitch having a significantly lower softening point. It is generally said that the spinning temperature is 40.degree. to 100.degree. C. higher than the softening point. It has, therefore, been difficult to spin a mesophase pitch having high anisotropy at temperatures lower than 300.degree. C. That is, in many cases, the mesophase pitch has been spun at temperatures as high as 340.degree. to 380.degree. C. At such high spinning temperatures, however, the mesophase pitch is liable to undergo thermal decomposition and a thermal condensation reaction, thereby producing gas and high molecular weight substances. Thus, it has been difficult to stably spin the mesophase pitch for long periods of time.
Various attempts have been made to overcome the above problems of the conventional mesophase pitch. Japanese Patent Publication No. 30192/84, for example, discloses a method of partially hydrogenating a mesophase pitch to appropriately weaken its laminated state and then spinning it as an isotropic pitch. Japanese Patent Application (OPI) No. 18421/83 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") discloses a method utilizing a specific premesophase pitch which is isotropic at the time of spinning but is converted into an anisotropic state at the stage of carbonization. In any method, however, the pitch is spun at an isotropic stage where the molecular orientation is poor. Therefore, inevitably, the orientation of molecules in fibers is poor as compared with that in an anisotropic pitch. Moreover, it cannot be said to be advantageous from an industrial standpoint to hydrogenate a viscous and carbonaceous material in which condensed polycyclic aromatic units are laminated.
Coal tar, naphtha tar or a fluid catalytic cracking residue of a petroleum fraction contains inorganic substances such as free carbon and catalyst powder. These substances not only make an obstacle to the spinning of the pitch, but also if contained in fibers in the form of fine particles, produce defects in the fibers and weaken their strength. Therefore, many methods of removing such inorganic substances have been developed. Japanese Patent Application (OPI) No. 167788/81 (corresponding to U.S. patent application Ser. No. 143,136 filed on April 23, 1980), for example, discloses a method which comprises subjecting a catalytic cracking residual oil to thermal soaking to obtain a pitch, extracting the pitch with a solvent to remove insoluble ash components such as cokes and finely divided catalyst particles, and then treating the resulting mass with an anti-solvent compound to precipitate an anisotropic pitch precursor. Japanese Patent Application (OPI) No. 164386/84 discloses a method comprising subjecting a coal tar pitch to refining by a two-step thermal modification wherein at the first step, a thermal modification is lightly performed until a small amount of mesophase spheres are formed, and at the second step finely divided free carbon having a size of not more than 1 micron and inorganic substances constituting an ash are all removed together with mesophase spheres by techniques such as filtration. In accordance with these methods, however, it is difficult to also remove submicron particles and, thus, the strength and the modulus of elasticity of carbon fiber cannot be increased.